B-lymphocyte cell lines were established using Epstein-Barr Virus from peripheral blood of normal volunteers, juvenile-onset diabetics and patients with extreme insulin resistance. Several patients with extreme insulin resistance have been found to have decreased numbers of insulin receptors on cultured lymphocytes. These syndromes have included leprechaunism, Type A extreme insulin resistance, and the Rabson-Mendenhall syndrome. Another patient with leprechaunism has been found to have a normal number of receptors. However, the receptors in this patient have a markedly decreased sensitivity to changes in temperatures and pH.